The present invention relates to arrayed waveguide gratings (AWGs), and more particularly to AWG arrangements with reduced channel passband asymmetry.
AWGs, sometimes also known as “phasars” or “phased arrays”, are well known components in the optical communications network industry. An AWG is a planar structure comprising an array of waveguides arranged side-by-side which together act like a diffraction grating. AWGs can be used as multiplexers and as demultiplexers, and a single AWG design can commonly be used both as a multiplexer and demultiplexer. The construction and operation of such AWGs is well known in the art. See for example, “PHASAR-based WDM-Devices: Principles, Design and Applications”, M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, No. 2, 20 Jun. 1996, and U.S. Pat. No. 5,002,350 and WO97123969, both incorporated by reference herein.
FIG. 1 illustrates the layout of a conventional AWG. It comprises a substrate (“die”, “chip”) 100 supporting one or more input optical waveguides 110 delivering optical energy into an “input slab” region 112. The slab region is a planar waveguide which confines the input optical energy in only the vertical dimension; the energy is permitted to spread transversely without restriction. The input slab is sometimes referred to herein as an “input free space region”, or an “input free propagation region”. Note that despite the implication of these terms, energy spreads freely in these regions only in the transverse dimension; it remains confined vertically.
An image of the input optical energy (or an interference pattern, if there is more than one input optical waveguide) is developed on the far boundary 114 of the input free space region 112. At this boundary the light enters the input end 116 of a waveguide array 118 which consists of tens or hundreds of individual waveguides. The array waveguides are of lengths which increase linearly across the array, each waveguide having a length which differs from its nearest adjacent waveguide by a value ΔL.
Optical energy exits the waveguide array 116 at an output end 120 thereof, and delivers the light into an “output slab” region 122. Like the input slab, the output slab region is a planar waveguide which confines the input optical energy in only the vertical dimension. The energy is permitted to spread transversely without restriction, and for that reason the output slab is sometimes referred to herein as an “output free space region”, or an “output free propagation region”. In some embodiments the input and output free space regions overlap each other such that the input and output beams cross each other.
A diffraction pattern is developed on the far boundary 124 of the output free space region 122, where the light enters a set of one or more output optical waveguides 126. The structure can be used as a demultiplexer if there is only one input waveguide 110 and more than one output waveguide 126; in this case information can be carried on multiple channels (wavelengths) in the single input waveguide and the channels are separated out by the AWG for delivery into the different output waveguides. The structure can also be used as a multiplexer if operated in reverse. It can furthermore be used as a router if there are multiple input waveguides 110 and multiple output waveguide 126.
A problem that arises in AWG designs is that the passband spectrum of a given channel often can be asymmetrical in shape. Depending on the shape of the filter curve—“flat top” or “Gaussian”—the asymmetry may manifest itself in various forms. FIG. 2 illustrates this asymmetry for an AWG with flattened wavelength response (a so-called “Flat top” AWG). It can be seen that the passband asymmetry is most visible at the top of each filter passband shape, which appears slanted instead of flat. A typical example is shown more symbolically in FIG. 3. The response shown in FIG. 2 is from a 16×50 GHz Cyclic flat top AWG. It can be seen that the tilting of the top of the filter shape varies as a function of the channel number (i.e. as a function of wavelength).
The asymmetry referred to herein is asymmetry of the response about the intended center wavelength of a given channel. This asymmetry can be quantified in a number of different ways, but for purposes of the present description, for flat top devices, we quantify it in dB/nm as the slope of a straight line through two points spaced evenly around the central wavelength of the channel (see FIG. 3). This slope is sometimes referred to herein as the “tilt” of the response curve for the particular channel. For the spectrum shown in FIG. 2 the corresponding asymmetry values are given in FIG. 4. It can be seen that the asymmetry in this case varies from approximately −4 dB/nm to approximately 4 dB/nm.
For arrayed waveguide gratings with a Gaussian filter shape (Gaussian AWGs), the “tilt” is not evident in the response curve for a particular channel. Instead, asymmetry manifests itself as a channel dependent wavelength shift of the channel positions. FIG. 5 illustrates the response of a particular channel in a Gaussian AWG. The dotted line in FIG. 5 illustrates the intended (ideal) response curve and the solid line illustrates the response curve in which the asymmetry is included. The center wavelength shift arising due to the asymmetry is indicated as δλ. Again, while a number of different ways can be used to quantify the asymmetry for Gaussian AWGs, for purposes of the present discussion it is quantified as equal to the channel dependent wavelength shift. The asymmetry for an example Gaussian AWG is shown in FIG. 6. The device represented in this figure is a 40×100 GHz Gaussian AWG. On the horizontal axis the values 5 and 44 for CHANNEL_ID correspond with ITU frequencies of 192.1 THz and 196.0 THz respectively. On the vertical axis the difference between the channel positions and the ITU frequencies is given in nm. It can be seen that the asymmetry in this case varies from about −0.005 nm to about 0.005 nm.
It would be desirable to reduce or eliminate the channel passband asymmetry observed in conventional AWGs.